1. Field of the Invention
The present invention relates to circuit board devices which are preferably mounted on motherboards together with electronic components or other elements subassembled beforehand on the circuit board devices and also relates to a mounting method therefor.
2. Description of the Related Art
In general, as circuit board devices, for example, the structure in which a plurality of electronic components is subassembled beforehand on a module substrate has been well known (for example, disclosed in Japanese Unexamined Patent Application Publication No. 2001-127188).
The circuit board devices described above according to a conventional technique each includes a substrate having a front surface on which electronic components are mounted and a rear surface which are to be mounted on a motherboard, a plurality of end-surface electrodes provided on end surfaces of the substrate at predetermined intervals and connected to the electronic components, and a plurality of lands which are provided at the rear surface and connected to the plurality of end-surface electrodes and which are to be soldered to related electrodes of the motherboard together therewith.
In the circuit board device described above, the end-surface electrodes are each formed, for example, of a strip-shaped metal film extending in the thickness direction of the substrate, and one end of the end-surface electrode is connected to the electronic component through a wire pattern or other connection provided at the front surface. In addition, the other end of the end-surface electrode extends to the rear surface side and is integral with the land made of a metal film or other suitable material. In this case, the land is formed to have a sufficiently large area so as to be securely soldered to the motherboard side.
When the substrate as described above is mounted on a motherboard, a step of applying solder to the end-surface electrodes and lands is first performed before the substrate is mounted. In this solder-application step, after a predetermined amount of solder is adhered to each of the end surface electrodes, lands, and other elements of the substrate and is then melted, the solder is solidified in a protruding state in which the solder hangs downward from the land by its surface tension and its own weight.
Next, when this substrate is placed on the motherboard and is then heated while the protruding end of the solder is in contact with the motherboard side, the solder is melted and distorted between each land of the substrate and the motherboard, and as a result, the lands and the motherboard are soldered to each other. In this step, even when the substrate is deformed, for example, by warping, since the solder is sequentially melted and distorted, all solder is brought into contact with the motherboard side, and as a result, when the substrate is mounted thereon, the warping or other defects of the substrate can be absorbed by the solder in the protruding state.
According to the conventional technique described above, lands each having a sufficiently large area are formed on the substrate and are then soldered to the motherboard side, and hence the substrate can be securely mounted on the motherboard.
However, when the land having a large area is formed, in a solder-application step, since the solder expected to protrude toward the motherboard side spreads along the land to form a thin film, it is necessary to adhere a large amount of solder to the land. Otherwise, due to an insufficient protruding dimension of the solder, the warping or other defect of the substrate may not be absorbed in some cases, and as a result, connection defects of the land are likely to occur.
On the other hand, when a large amount of solder is adhered to the land, the consumption of the solder is unnecessarily increased, and in addition, short-circuiting may occur between adjacent lands by the solder which is melted and distorted between the land and the motherboard in a substrate mounting step. As a result, the increase of the amount of the solder has been limed to a certain level.
Accordingly, in the conventional technique, since it has been difficult to simultaneously obtain an appropriate land area and an appropriate protruding dimension of the solder, there have been problems in that the substrate cannot be mounted on the motherboard with a sufficient bonding strength or that the connections with the lands or other elements become unstable, resulting in degradation of reliability.